Charged particle beams, such as focused ion beam systems and electron beam systems, direct charged particles onto a work piece for processing the work piece by, for example, milling or forming an image of the work piece. Charged particle beam systems are used, for example, in integrated circuit fabrication and other nanotechnology processing.
Charged particles beam systems typically include a source of particles, a beam blanker, accelerating lenses, focusing optics, and deflection optics. A charged particle source may be, for example, a liquid metal ion source, a plasma ion source, or a thermal field electron emitter, such as a Schottky emitter. A beam blanker interrupts the beam by directing it away from the work piece and into a solid stopping material.
The focusing optics focus the beam into a spot or a predefined shape on the surface of a sample. Focusing optics typically include a combination of condenser lenses and an objective lens. The lens can be electrostatic, magnetic, or various combinations of the two. Charged particle lenses, like light lenses, have aberrations that make it difficult to focus the charged particles to form a sharp image. The aberration is least for charged particles passing through the center of the lens, and the aberration increases as the distance from the center of the lens increases. It is desirable, therefore, for the charged particle beam to pass very near the center of the lens. One type of aberration, referred to as “beam interaction” occurs because the particles in the beam, all having the same electrical charge, repel each other. The closer the particles are to each other, the greater the repulsive force. Because the particles are typically converging after passing through the objective lens, it is desirable to position the objective lens as close as possible to the work piece, to reduce the time that the particles are focused in a tight beam. The distance between the objective lens and the work piece is referred to as the “working distance.”
The deflection optics direct the beam to points, referred to as “dwell points” or “pixels,” on the surface of the work piece. For example, the beam may be directed in a raster pattern, in a serpentine pattern, or toward an arbitrary sequence of individual points. The beam will typically dwell at a point for a specified period, referred to as “dwell period,” to deliver a specified “dose” of charged particles, and then be deflected to the next dwell point. The duration of the dwell period is referred to as the “dwell time” or the “pixel rate.” (While pixel “rate” more properly refers to the number of pixels scanned per second, the term is also sometimes used to indicate the time the beam remains at each pixel.)
The deflection optics can be magnetic or electrostatic. In focused ion beam systems, the deflection optics are typically electrostatic. Electrostatic deflectors for focused ion beams are typically octupoles, that is, each deflector includes eight plates, distributed around the circumference of a circle. Different voltages are applied to the eight plates to deflect the beam away from the optical axis in different directions.
If the deflector is placed below the objective lens, the beam can pass through the center of the objective lens to minimize aberration. Such a configuration is used, for example, in some VisION Systems sold by FEI Company, the assignee of the present invention. Placing the deflector below the objective lens, however, increases the working distance, thereby increasing the beam aberration.
To minimize the working distance, the deflector can be placed above the objective lens. With the deflector above the lens, however, when the beam is deflected, it is moved away from the center of the lens, thereby increasing certain aberrations. To solve this problem, many focused ion beam systems use a pre-lens two-stage deflector 100 as shown in FIG. 1 to deflect a beam 102 from an optical axis 104. A first stage 110 deflects the beam 102 to one side of optical axis 104, and the second deflector 114 deflects the beam back to the other side of optical axis 104 so that the beam 102 passes through the center of an objective lens 120, but at an angle such that the beam is deflected to be in the correct position as it impacts a work piece 122. Voltages of the same magnitude are typically applied to both stages of the deflector to achieve the desired deflection.
Charged particle beams process work pieces by delivering a calculated number of particles to precise locations on the work piece. Each particle causes a change in the work piece and the ejection of secondary particles. To precisely control the processing, whether for milling or for imaging, one must control the number of particles impacting each point on the surface. As features of the work pieces processed by charged particle beams get ever smaller, charged particle beams must be able to more precisely deliver a controlled number of charged particles to each small point on the work piece surface. This precise control requires deflectors that can rapidly move a beam from pixel to pixel, while delivering the correct dose of particles to each pixel.
A significant problem occurs due to the fact that the two deflectors in the typical pre-lens deflector, referred to as an upper octupole and a lower octupole, are separated by a distance that is typically many millimeters. Because FIB ions, such as gallium ions, are relatively massive, the time it takes ions to traverse the distance between deflectors is non-negligible in comparison to short pattering dwell times. As a result, when a signal applied to a deflector system is changed to direct the beam from a first dwell point to a second dwell point, charged particles that have already passed through part of the deflection system when the voltage is changed (the upper octupole) will not receive the correct forces at the lower octupole. This will cause the charged particles to be directed to points other than either the first or the second dwell point. As dwell periods become shorter, voltage changes become more frequent, and the number of particles that are traversing the deflection system during voltages change increases, so more particles are misdirected, making it impossible to precisely process a work piece.
If the same wave-forms are applied to both deflectors, there will be “timing errors” due to the time-of-flight (TOF) it takes for the ions to travel from one deflector to the other. This leads to patterning errors that generally manifest themselves as over-shoot effects. The patterning errors are particularly obvious at lower landing energies and short dwell times. FIG. 2 shows the milling path 202 for a focused ion beam system with a beam energy of 8 kV and a 300 nanosecond dwell time. Line 204 shows the intended beam path. Significant overshoot effects can be clearly seen wherever the beam changed direction.
These types of patterning errors are particularly problematic for circuit edit and beam chemistry applications. For example, in many circuit edit applications, very tight geometry is involved with little room for error. Also, gas-assisted etching is often required for the necessary high-aspect ratio milling involved. When using gas-assisted etching, short dwell times are needed to avoid depletion of the etching gas (with resulting degradation of milling performance) at a particular location. Because of the low tolerances involved in circuit edit, milling outside the desired beam path (as shown in FIG. 2) can result in damage to essential circuit features.
One technique for dealing with time-of-flight timing errors is described in U.S. Pat. No. 7,569,841 to Hill et al., for “Deflection Signal Compensation for Charged Particle Beam,” which is assigned to the assignee of the present application and incorporated herein by reference. Hill describes a scheme that involves delaying the timing of deflection signals that are applied to the two deflector systems by an amount related to the transit time between the deflectors. This methodology works quite well but has the drawback of requiring an extra set of amplifiers (typically eight outputs) to drive an extra deflector, as well as a means of generating time-delayed versions of the deflection signals. The extra set of amplifiers results in extra deflection noise, as well as adding extra cost and complexity to the ion beam system.
Accordingly, there is a need for an improved method of correcting for TOF errors that avoids these problems of the prior art.